1. Field of Invention
This invention relates to wireless systems using Code Division Multiple Access (CDMA) systems and methods of operation. More particularly, the invention relates to wireless CDMA systems which employ multiple antennas at the receiver.
2. Description of Prior Art
In CDMA systems, any processing at the receiver that reduces interference improves link quality in terms of reduced noise and system capacity in terms of increased numbers of users (termed mobiles) served by the system. Interference reduction is thus an important objective for CDMA systems. However, before signal processing can occur in a CDMA receiver, the timing of each received signal must be accurately estimated to allow despreading to occur. The process of estimating the timing of each received signal is termed synchronization. Previously proposed signal processing methods which reduce interference assume that synchronization has been accomplished prior to processing. However, estimating the time delay for all received signals becomes more difficult due to the increased interference as the number of received signals increase.
One signal processing method which reduces interference and increases system capacity in CDMA systems is the use of multiple antennas or antenna arrays. Antenna arrays can provide diversity reception (termed diversity gain) as well as improvement in average Signal-to-Noise Ratio (termed aperture gain) to make the wireless link more robust in the presence of multipath fading. Antenna arrays can also achieve interference rejection (Signal-to-Interference-Plus-Noise or SINR gain) through appropriate antenna combining. Capacity improvement is accomplished by combining the weighted outputs of each antenna in a way that cancels the interference and/or enhances the desired signal. The weights are computed (and tracked in real time if necessary) according to an optimization criterion and an associated adaptive algorithm. Typical algorithms require an estimate of an antenna cross-correlation matrix for each signal being received. The cross-correlation matrix contains information concerning the interfering signals (e.g., power and angle-of-arrival) which is useful in calculating the appropriate antenna weights to reduce the interference. In CDMA systems, the cross-correlation matrix is typically formed using post-correlated signals (i.e., after despreading), since estimating the matrix before correlation (pre-correlation) involves calculating vector outer products at the chip rate which requires high-speed computation. On the other hand, calculating the post-correlated cross-correlation matrix requires synchronization to occur previously, which again requires improved signal acquisition techniques. What is needed in the art is a CDMA system and method of operation which provides an improved synchronization technique for interference reduction in signal acquisition and has reasonable computational complexity in processing pre-correlated signals.
Prior art related to reducing interference in CDMA systems includes the following:
U.S. Pat. No. 5,500,856 entitled xe2x80x9cTransmission De-Correlator For Eliminating Interference In A CDMA Communication Systemxe2x80x9d, issued Mar. 19, 1996 discloses a transmission de-correlator having a memory storing a set of data representative of the products of transmission information bits for the respective channels, and the sums of the products of the spread code data sequence for the respective channels, and the elements of an inverse matrix with respect to a predetermined matrix of a cross-correlation between the spread code data sequences for the respective channels. The sums correspond to additions of the products along a direction of rows of the inverse matrix. A counting device is operative for causing the memory to sequentially output the data therefrom. An adder serves to calculate a sum of the data outputted from the memory for the respective channels.
U.S. Pat. No. 5,568,473 entitled xe2x80x9cMethod And Apparatus For Simple And Efficient Interference Cancellation For Chip Synchronized CDMAxe2x80x9d, issued Oct. 22, 1996 discloses canceling interference from other users by constructing a single reference sequence which is representative of the spreading sequences of all users in the system. Interference from other users can be cancelled without each receiver employing a separate decorrelator for every other user in the system and having an accurate estimate of the received power level of every transmitted signal.
None of the prior art discloses CDMA systems and methods of operation using antenna arrays with improved synchronization for interference reduction in signal acquisition and reasonable computational complexity for both signal acquisition and signal processing.
An object of the invention is a CDMA system and method of operation using one or more antenna arrays for interference reduction and having improved signal acquisition and signal processing with reasonable computational complexity.
Another object is a receiver and method of operation having improved synchronization for signal acquisition and signal processing in a CDMA system including one or more antenna arrays.
Another object is a receiver and method of operation in which a universal inverse cross-correlation matrix is used in both signal acquisition and signal processing for reduced computational complexity in a CDMA system including one or more antenna arrays.
Another object is a receiver and method of operation in which a universal inverse cross-correlation matrix array facilitates synchronization in signal acquisition and reduced computational complexity for both signal acquisition and signal processing in a CDMA system having one or more antenna arrays.
These and other objects, features and advantages of the invention are achieved in a Code Division Multiple Access system and method of operation which provide reduced interference for received signals and improved signal acquisition and processing with reduced computational complexity. The system includes a base station coupled to an antenna array of at least two or more antennas and serving a plurality of users. A receiver in the base station includes a universal inverse cross-correlation matrix coupled to the antenna array, a signal acquisition and a signal processing circuit serving each user. Each signal acquisition circuit comprises a series of delay stages in which the incoming antenna signals in each stage are correlated with a spreading code and combined in a multiplier coupled to the universal inverse cross-correlation matrix which facilitates improved time delay estimation for signal acquisition. Each multiplier combines the correlated signals of the stage with the output of the universal inverse cross-correlation matrix to provide a signal amplitude representative of the signal energy in an antenna path for a given time period, with individual delays separated by a half of chip period. The amplitudes for each of the delay stages are captured in buffers which contain threshold information for selection of the strongest received signal. The signal processing circuit combines the strongest received signal with a channel estimate and the universal inverse matrix output in a multiplier to provide an output signal for demodulation and decoding with improved signal quality due to (a) reduced interference, (b) improved time delay estimation for signal acquisition and processing, and (c) the universal inverse cross-correlation matrix reducing computational complexity in signal acquisition and signal processing.